The invention relates to an adaptive balance filter.
One substantial problem in digital communications systems is duplexing, that is simultaneous transmission and reception at high data rates over existing two-wire lines. Difficulties arise due to crosstalk from the analog transmission route to the analog reception route, above all in dividing the two-wire lines into transmission and reception routes. Instead of the previously typical analog hybrid network technology, a method generally known as digital echo suppression has gained wide use, especially in public systems with ranges of about 8 km. The assumption is that crosstalk can be described by a transfer function of the echo path, corresponding to a filter between the transmission and reception routes. For echo suppression, a so-called balance filter is incorporated between the transmission and reception routes, which has a transfer function that is inverse to the echo path.
Such a balance filter is known, for instance, from a paper by S. J. Poole, G. Surace, B. Singh and N. P. Dyer, entitled A CMOS Subscriber Line Audio Processing Circuit Including Adaptive Balance, from the International Symposium on Circuits and Systems, Helsinki, June 7-9, 1988, pp. 1931-1934 in particular FIG. 4. That balance filter is constructed as an adaptive filter. In other words, the balance filter adapts to various or varying transfer functions of the echo path. In order to set the inverse transfer function, the coefficients of the balance filter are varied so that an error signal, which is formed in accordance with certain criteria, is minimized. The known adaptive balance filter includes, among other elements, an adaptive filter (background filter), which ascertains the optimal filter coefficients from the error signal, and a shadow filter (foreground filter) with fixed filter coefficients. The filter coefficients of the shadow filter are set equal to the filter coefficients of the adaptive filter, under a certain condition. The condition for adoption of the filter coefficients is that over a given relatively long period of observation, the error of the adaptive filter is less than the error of the shadow filter. If a transition signal is missing adaptation is in fact virtually impossible, and the transfer function of the adaptive filter therefore involves major error. In such a state, the most recently set transmission behavior of the shadow filter is therefore kept constant, that is no filter coefficients are adopted.
Duplexing with narrow band signals is still problematic, which is true for data transmission by modem, for instance. On one hand, adaptation of the adaptive filter with narrow band signals is very difficult and entails major error. On the other hand, adoption of the flawed coefficient of the adaptive filter by the shadow filter takes place continuously, because initially the shadow filter has a greater error than the adaptive filter. However, since both the adaptive filter and thus the shadow filter involve major error, adequate echo suppression is not possible, and that in turn can lead to interference in data transmission.